The genetic and molecular mechanisms controlling the regular and precise segregation of chromosomes in mitosis and meiosis are unknown. This research is directed toward a combined genetic and biochemical characterization of aneuploid mitotic chromosome behavior in yeast, a simple, easily manipulated eucaryote that can serve as a model system. Three coordinate approaches will be employed. (1) An attempt will be made to isolate and characterize the complete set of single disomic haploids in yeast. The mitotic stability of each disome will be determined, and relationships among individual aneuploid chromosomes in fostering mitotic stability or instability will be explored. (2) The disomic set will be employed in a characterization of the size of individual chromosomal DNA molecules. (3) The disomes isolated in this study will provide genetic tools for the isolation and characterization of mutations conditionally defective in mitotic segregational control.